Magnesium is a preferred structural material for mass-sensitive applications because of its low density. However magnesium is also chemically active and will readily react with water and oxygen and corrode.
An approach to minimizing the high tendency for magnesium to corrode is to develop an adherent protective coating which will limit access of the corrosive environment to the magnesium metal. Suitable coatings, not all of which are equally effective, may be formed either chemically or electrochemically on the surfaces of magnesium articles and are variously described as passivation coatings, conversion coatings and anodized coatings. These are usually applied through batch or continuous processes conducted by immersion of the treated components in large baths of the appropriate chemicals.
Alternatively, and particularly where it is desired to also convey a pleasing appearance to the article, its surfaces may be painted. In many cases, multiple paint layers may be employed. These layers may be multiple layers of the same paint formulation with multiple coats employed to achieve a desired film thickness. More commonly, the layers will be of differing formulations to convey attributes to the overall painted surface which are not readily combined in a single coating.
An issue common to all of these coatings, however, is that they suppress corrosion through creating a barrier between the magnesium and the chemically active environment. Thus, rupture of a small region of the film such as would result from a scratch or a stone chip will render it ineffective in suppressing corrosion in the vicinity of the rupture. Worse, since corrosion is electrochemical, a scratch or rupture of the film will create a small area which will be anodic to a much larger cathodic area and accelerate and intensify the corrosion at the scratch or rupture.
An exposed magnesium surface may be re-covered with a protective coating to repair or regenerate the coating in the scratched or stone-chipped region. However, the original process used to apply the coating to a component initially may well be unsuited or less suited for repair when the component is part of an assembly or subassembly. Further, because of the highly localized corrosion which results, prompt repair of the coating is required and even the time between the rupture of the coating and the time the rupture is detected might be sufficient to generate unacceptable corrosion.
Thus there is a need for a process for easily and conveniently promptly re-establishing a corrosion-inhibiting coating on magnesium after rupture of a pre-existing film.